(1) Field of the Invention
The present invention relates to substrates for magnetic discs, crystallized glass suitable for such magnetic discs, magnetic discs and a process for producing magnetic disc substrates.
(2) Related Art Statement
Principal constituent elements of a magnetic memory apparatus such as a computer are a magnetic recording medium and a magnetic head for magnetically effecting recording and reproduction. As the magnetic recording medium, the flexible disc and the hard disc are known. As to the hard disc, an aluminum alloy has been principally used as its substrate material. However, with the miniaturization of the hard disc drive, a floating amount of the magnetic head conspicuously decreases. Owing to this, extremely high accuracy has been demanded for the flatness and smoothness of the surface of the magnetic disc.
In general, it is necessary to suppress the maximum vertical height of an uneven surface of the magnetic disc to a level not more than half the floating amount of the magnetic head. For example, if the hard disc drive has an allowable floating amount of, for example, 75 nm, the maximum vertical of the uneven surface of the disc must be not more than 38 nm. Particularly, it has been recently required that the center line average surface height (Ra) of the uneven surface of a reading/writing zone of the magnetic disc substrate is not more than 20 .ANG.. However, the aluminum alloy substrate has a small hardness. Therefore, even though the substrate is polished with use of high precision grinding grains and working machines, the polished surface of the substrate is plastically deformed. Consequently, it is difficult to produce a flat and smooth surface at a high accuracy greater than a given degree. For example, even if the surface of the aluminum alloy substrate body is plated with nickel/phosphorus, the flat and smooth surface cannot be be obtained at the above level.
Further, as the hard disc drive is miniaturized and made thinner, it has been demanded that the thickness of the magnetic disc substrate is made smaller. However, since the aluminum alloy has low strength and low rigidity, it is difficult to maintain the given strength required from the specification of the hard disc drive while making, the disc thinner. Particularly, if the magnetic disc substrate is not more than 0.5 mm, the strength of the substrate becomes insufficient, so that the substrate may be warped or vibrated during high speed rotation and during the start of rotation.
Magnetic resistance type heads (MR head) had recently begun to be used, and demand for reducing noise of the magnetic disc has been increasing. In order to reduce such noise, it is known to thermally treat a magnetic film when or after the magnetic film is sputtered. In order to effectively reduce the noise of the magnetic disc by the thermal treatment, the thermal treatment needs to be effected at a temperature not less than 280.degree. C. However, the thermal treating temperature cannot be raised to more than 280.degree. C. in the case of the aluminum alloy substrate.
As the hard disc drive becomes more compact and thinner, there is a strong demand to decrease the thickness of the substrates for the magnetic discs. However, since the aluminum alloy has low strength and low rigidity, it is difficult to make the discs thinner while the given strength required from the specification of the hard disc drive is being maintained. In particular, if the magnetic disc drive is worked to not more than 0.5 mm, there is problems that the substrate is warped or the surface of substrate is vibrated during a high speed rotation or at the time of starting owing to insufficient strength of the substrate.
In order to solve the above problem, magnetic disc substrates made of glass have been put into practical use in some cases. However, since particularly high strength is required in the case of the substrate for the HDD magnetic disc, chemically tempered glass or glass ceramics need to be used. When such a glass material is used, a magnetically recording surface having a very small Ra of not more than 20 .ANG. can be formed.
Since glass actually has a low strength, it does not have reliability sufficient enough to be used as substrates for HDD type magnetic hard discs. Chemically reinforced glass such as soda-lime glass is also known. However, if such a chemically tempered glass is used as a substrate for magnetic disc, alkaline metal ions contained in the substrate may be dissolved out to corrode a magnetic film.
On one hand, in order to prevent the head sticking phenomenon that the magnetic head slider is stuck to the surface of the magnetic disc when the magnetic disc drive is stopped, it is necessary to form such a texture on the surface of the magnetic disc that is constituted by bumps having heights of around 200 .ANG.. On the other hand, as mentioned above, it is necessary to attain the flatness and smoothness of the surface to a high degree. For this reason, a reading/ writing zone and a landing zone are formed on the surface of the magnetic disc, and the reading/writing zone is finely polished to increase the magnetically recording density, whereas the texture is formed on the landing zone.
The above texture is formed by a photography process or an etching process in the case of a soda lime glass (chemically tempered glass). However, these process are costy, and it takes a larger cost so as to particularly decrease the area of tips of the protuberances constituting the texture.
Further, in the case of the aluminum substrate for the magnetic disc, a metal having a low melting point is sputtered upon the surface of the substrate, which is heated to form minute, semi-spherical protuberances. However, it is difficult and takes a large cost to form such protuberances only upon the landing zone.
Furthermore, as to a magnetic disc substrate made of crystallized glass, a texture is formed through polishing by utilizing difference in hardness between crystalline grains and the intergranular phase constituting the crystallized glass. However, according to this process, the texture was formed on both the landing zone and the reading/writing zone but it was impossible to form the texture on the landing zone alone.